#62 Environmental impacts of lower speed limits

The benefits of lower vehicle speeds are regularly reported in terms of safety, particularly for vulnerable road users, where the relationship between impact speed (velocity) and survivability is generally well-established, as noted previously. Less often reported are the environmental impacts of lower speeds, including reduced vehicle emissions and fuel consumption, which may be expected in many cases where speed reduction measures are implemented. With research on this topic relatively scarce (compared with safety-focused studies) and variable results reported, this brief summarises key findings of a recent systematic review of literature on the effects that speed limit changes have on vehicle emissions[1]. Establishing the rationale, this study reports that the transport sector contributes 39% of nitrogen oxides, 26% of black carbon, 20% of carbon monoxide, and 10% of particulate matter in overall (European) greenhouse gas emissions.

Of the key findings reported, it is first critical to note the importance of contextual factors, which largely determine the outcomes, among which is the concentration of research from higher-income countries. As stated by the authors:

The extent of these (emissions) reductions is highly variable and contingent upon the specific speed limits or limit reductions, the local context, the vehicle type, and the baseline types and levels of pollutants.

Different perspectives, variable outcomes

The topic of vehicle emissions relative to speed is a contested space, according to the cited study. Researchers have argued that certain factors contribute significantly to emissions at both lower and higher speeds, as summarised below in Table 1. Further, as road design and vehicle standards continue to evolve, and electric vehicle use increases, this space is in a state of constant change.

Table 1: Factors contributing to vehicle emissions at higher and lower speeds

The systematic review found mixed evidence regarding low-speed areas, with some studies indicating that increased emissions can result from reduced speeds. In higher-speed areas, the findings are more conclusive and consistent that ‘reduced speed limits correlate with significant reductions in NOx, CO2, and particulate matter emissions’. It is also notable that reducing speeds in urban areas may also have a less direct but nonetheless significant impact on emissions by encouraging a modal shift away from private vehicle use.

Importantly, findings also point to driver-related factors that increase emissions, particularly in lower speed zones, including aggressive acceleration and deceleration:

Therefore, enhancing the smoothness of acceleration and deceleration manoeuvres at low speeds could be more effective in reducing emissions than simply lowering the average speed.

[1] Fondzenyuy, S., Turner, B., et al. (2024). The Impact of Speed Limit Change on Emissions: A Systematic Review of Literature. Sustainability 16(17), 7712.